What would cause leaves to sprout growths?

Leaves with club-like appendages may have spindle gall. It’s harmless, even if it is ugly. Galls are created by a variety of insects and mites, as well as fungi, bacteria, and other organisms. Mites that feed and breed on leaves cause the plant tissue to distort and form a hollow structure that expands as the leaves grow. The female mites lay numerous eggs inside and feed on the softer new growth of the gall. When the young mature, they exit the gall to start the cycle over again on new leaves. Different species of mites go after different plants; some prefer viburnum, some birch, while others have a taste for maples. Spindle gall grows around the Eriophyid mites, providing a home for generations of viburnum connoisseurs. These mites are microscopic beasties a quarter of a millimetre long that hitch a ride on birds, other insects, or even the wind.

It’s pretty hard to do battle with an unseen enemy, so don’t try. In severe cases, apply a dormant oil spray in early spring to the trunk or branches, where the female mites overwinter. But it’s difficult to get the timing right. Chances are that if they have arrived at your cottage, the mites and the galls have taken up permanent residence.